The present invention relates in general to data storage systems. In particular, the present invention relates to a method and apparatus for positioning a transducer using a phase difference in surface profile variations on a storage medium.
A typical magnetic data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
The actuator assembly typically includes a coil assembly and a plurality of outwardly extending arms having flexible suspensions with one or more transducers and slider bodies being mounted on the suspensions. The suspensions are interleaved within the stack of rotating disks, typically using an arm assembly (E-block) mounted to the actuator assembly. The coil assembly, typically a voice coil motor (VCM), is also mounted to the actuator assembly diametrically opposite the actuator arms. The coil assembly generally interacts with a permanent magnet structure, and is responsive to a transducer positioning controller.
In a typical digital magnetic data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One of the information fields is typically designated for storing data, while other fields contain track and sector identification and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers which follow a given track and may move from track to track, typically under servo control of a position controller.
The head slider body is typically designed as an aerodynamic lifting body that lifts the transducer off the surface of the disk as the rate of spindle motor rotation increases, and causes the transducer to hover above the disk on an air-bearing cushion produced by high speed disk rotation. The separation distance between the transducer and the disk, typically 0.1 microns or less, is commonly referred to as head-to-disk spacing.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field.
Conventional data storage systems generally employ a closed-loop servo control system to move the actuator arms to position the read/write transducers to specified storage locations on the data storage disk. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read servo information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking).
A servo writing procedure is typically implemented to initially prerecord servo pattern information on the surface of one or more of the data storage disks. A servo writer assembly is typically used by manufacturers of data storage systems to facilitate the transfer of servo pattern data to one or more data storage disks during the manufacturing process.
In one known servo technique, embedded servo pattern information is written to the disk along segments extending in a direction generally outward from the center of the disk. The embedded servo pattern is thus formed between the data storing sectors of each track. It is noted that a servo sector typically contains a pattern of data, often termed a servo burst pattern, used to maintain alignment of the read/write transducers over the centerline of a track when reading and writing data to specified data sectors on the track. The servo information may also include sector and track identification codes which are used to identify the position of the transducer. The embedded servo technique offers significantly higher track densities than dedicated servo, in which servo information is taken from one dedicated disk surface, since the embedded servo information is more closely co-located with the targeted data information.
In a further effort to increase disk capacity, a proposed servo information format was developed, termed pre-embossed rigid magnetic (PERM) disk technology. As described and illustrated in Tanaka et al, Characterization of Magnetizing Process for Pre-Embossed Servo Pattern of Plastic Hard Disks, I.E.E.E. Transactions on Magnetics 4209 (Vol. 30, No. 2, November 1994), a PERM disk contains embossed servo information in a number of servo zones spaced radially about the disk. Each servo zone contains pre-embossed recesses and raised portions to form a fine pattern, clock mark, and address code. The fine pattern and address code are used to generate servo information signals. To generate these servo information signals, the magnetization direction of the raised portions and the recesses must be opposite. The magnetization process involves first magnetizing the entire disk in one direction using a high-field magnet. Then, a conventional write head is used to magnetize the raised areas in the opposite direction.
While use of a PERM disk may increase disk capacity, such an approach suffers from a number of shortcomings. Servo information is provided on a PERM servo disk in a two-step magnetization process, as described above. This significantly increases the amount of time required to write servo information to the disk. Moreover, during the second step of the process, servo information is not yet available on the disk. Thus, an external positioning system must be employed, thereby increasing the cost of the servo writing process. Additional concerns associated with PERM disk technology include durability.
Finally, the PERM disk, like other embedded servo techniques, still stores servo information in disk space that could otherwise be used for data storage. As a result, PERM disk technology, although still at the research level, has not been widely accepted by industry.
Pre-embossed rigid thermal (PERT) disk technology uses the thermal response of a magnetoresistive (MR) head induced by servo information on a storage medium in order to position the MR head. As described in U.S. Pat. No. 5,739,972, issued Apr. 14, 1998 to Gordon J. Smith et al. and assigned to the assignee of the instant application, a PERT disk includes servo information provided to induce a thermal response in the MR head. The servo information is typically provided in the form of pre-embossed surface profile variations on the disk. A controller controls the relative position between the MR head and the embossed disk track using the thermal response induced in the MR head.
Typically in PERT disk technology, a read signal from an MR head is filtered to separate thermal and magnetic components. As disclosed in U.S. Pat. No. 6,088,176, issued Jul. 11, 2000 to Gordon J. Smith et al. and assigned to the assignee of the instant application, the thermal and magnetic components of a MR read signal are separated using a finite impulse response (FIR) filter. The thermal component is the thermal response of the MR head to the surface profile variations on the PERT disk. For the purpose of track following, for example, the surface profile variations may include serrated inner diameter (ID) and outer diameter (OD) track edges. For each track, the ID edge serration has a different serration frequency than the OD edge serration. By examining the frequency content of the thermal component of the read signal, the off-track direction and magnitude of the MR head can be determined and an appropriate control signal provided to the actuator to position the MR head over the centerline of a track. The frequencies may differ by a factor of two, for example, and alternate from track to track. Likewise, the serrations may be radially aligned, i.e., the serrations may be spaced further apart as one moves radially outward, as the serration frequencies relative to the MR head would be constant over the entire surface of the disk in a constant angular velocity system.
This two-frequency track serration arrangement provides improved track following without sacrificing data capacity of a disk. Unlike embedded servo techniques, this arrangement does not store servo information in disk space that could otherwise be used for data storage. However, the two-frequency track serration arrangement presents a number of disadvantages. The servo electronics required to examine the frequency content of the thermal component of the read signal and therefrom determine the off-track direction and magnitude of the MR head is relatively complex. In addition, PERT disks containing two-frequency track serrations are relatively difficult to manufacture. A similar two-frequency pit arrangement is disclosed in U.S. Pat. No. 5,251,082, issued Oct. 5, 1993 to Elliott et al. and suffers from analogous disadvantages. The Elliott et al. patent discloses the use of its two frequency pit arrangement to induce a magnetic read signal, i.e., no thermal component is utilized.
There exists in the data storage system manufacturing industry a need for an enhanced servo information format which is relatively easy to fabricate, and uses relatively simple servo electronics. The present invention addresses these and other needs.
The present invention is a method and apparatus for positioning a transducer relative to a storage medium in a storage device. The storage medium is moved relative to the transducer by a motor at a rated storage medium velocity. The storage medium has a plurality of tracks, each having a first edge and a second edge. The first edge and the second edge respectively comprise surface profile variations having a temporal frequency at the rated storage medium velocity. The surface profile variations of the first and second edges are phase modulated, i.e., have a phase difference relative to one another. First and second responses, e.g., thermal responses, are respectively induced in the transducer by the phase modulated surface profile variations of the first and second edges. The transducer may be positioned by a controller, for example, in response to at least one of the first and second thermal responses. A storage medium having phase modulated surface profile variations is relatively easy to fabricate. Moreover, relatively simple servo electronics can be used with the phase modulated surface profile variations.